1. Field of the Invention
The present invention generally relates to plasma enhanced semiconductor wafer processing systems and, more particularly, to an apparatus for measuring plasma characteristics within a semiconductor wafer process chamber.
2. Description of the Related Art
Plasma processing techniques are widely used in processing thin films on workpieces such as semiconductor substrates or wafers. For example, in plasma enhanced chemical vapor deposition (PECVD), the plasma provides a convenient means to enhance the reactivity of gas molecules in order to deposit films at low temperatures. In etch processing, exciting gaseous species into a plasma state greatly enhances the rate at which material layers may be etched, thereby increasing process throughput.
It is desirable to determine various properties of the plasma within the processing chamber, such as plasma density, charged-particle concentrations, and energy distribution functions. Analysis of such properties enables one to better control for example, the etch rate or deposition rate within the chamber.
An example of a conventional probe for monitoring plasma properties, also called a “Langmuir probe,” is shown in FIG. 1. A plasma enhanced, semiconductor wafer processing system 100 includes a plasma chamber 102 having chamber walls 112. The chamber 102 includes a pedestal 104 and an electrostatic chuck 105, for supporting a substrate 106, a gas inlet 108 for introducing process gases, an electrode 109 for energizing a plasma 128 with an energy source 110 having a frequency, in the range of, for example, between about 400 kilohertz (kHz) and about 500 megahertz (MHz). The energy source 110 is typically a radio frequency (RF) source. A conventional probe 120 includes a conductive electrode 122 surrounded by an insulating sheath 124. The probe 102 is positioned such that a probe tip 126 extends into the plasma 128. A data analyzer 130 is coupled to the probe 102 to analyze various plasma properties.
Unfortunately, because the probe tip 126 extends into the plasma 128, the probe tip 126 is subject to degradation from exposure to the plasma. Material from the probe tip 126 may be ejected or sputtered into the chamber 102, and the material may then deposit, for example, on the chamber walls 112, on pedestal 104, or even on the substrate 106. While material from the probe tip 126 that impinges upon the chamber walls 112 may be removed through a chamber cleaning process, material that impinges upon the substrate 106 will likely damage the structures thereon, resulting in the substrate 106 being discarded.
One may reduce the likelihood of having material ejected from the probe tip 126 damage a substrate being processed by inserting the probe 120 into the chamber 102 only during selected time periods. For example, one may choose to insert the probe 120 into the chamber 102 when no substrate 106 or when a dummy substrate is being processed. However, this approach fails to address the need for in-situ or “real time” information concerning the properties of the plasma as a substrate 106 is processed.
Acquiring in-situ information about the plasma is highly desirable because such information may reveal, for instance, when a plasma perturbation may have taken place within the chamber 102. A plasma perturbation may, for example, result in deleterious effects to the substrates being processed in the chamber 102 during or after the perturbation. Therefore, knowledge of the specific timing of a plasma perturbation could, in turn, aid in diagnosing which substrates 106 and how many substrates 106 may have been adversely affected. Specific causes of the perturbation, such as a damaged electrode, incorrect gas flow, arcing between the electrostatic chuck and the substrate, among other causes may also be diagnosed through in-situ monitoring of plasma properties.
Therefore, a need exists for a non-intrusive plasma probe that is capable of in-situ monitoring of plasma properties.